The present invention relates to electrical discharge machining components and, more particularly, to novel arrangements and guide bushings to be used with electrical discharge machines for machining work pieces, such as aircraft engine parts, which have several features machined therein.
Electrical discharge machining (EDM) is a well known process for forming features, such as holes, slots and notches of various shapes and configurations, in an electrically conductive work piece. Conventional EDM systems typically employ an electrode including a tip which is advanced toward the work piece. Suitable energy is supplied to the electrode from a power supply to create an electrical potential between the work piece and the electrode in order to create a controlled spark which melts and/or vaporizes the work piece to form the desired feature. The work piece can be submerged in a bath of dielectric fluid which assists in the formation of the spark discharges, cools the work piece during repeated spark discharges and carries away material eroded from the work piece.
Typically, as seen in FIGS. 1 and 2, a conventional EDM system includes an electrode 100 defining a longitudinal central axis “X”. Additionally, a bushing 104 can be provided near distal tip 108 of electrode 100. As seen in FIG. 2, bushing 104 defines a bore 110 extending therethrough which bore 110 is sized to permit passage of electrode 100 into, and through bore 110. Bore 110 includes a proximal portion 110a having a diameter substantially equal to the diameter of electrode 100 thereby defining a region of contact having a length “L1” (less than 10 mm) and a distal portion 110b having a diameter which is larger than the diameter of proximal portion 110a and electrode 100 thereby defining an annular flushing channel 112. In order to support the EDM process, fluid “F” (dielectric flushing fluid, e.g., de-ionized water) is injected and/or fed into flushing channel 112 by a feed channel 114. In this manner, during operation, fluid “F” can be injected along and towards distal portion 110b of electrode 110 to sweep and/or wash away debris. 
It is known to rotate, or spin, electrode 100 around longitudinal axis “X” while performing the EDM process, such that electrode 100 will wear uniformly and thereby form a substantially straight hole. A concern with this type of process is that during machining, rotation “R” of electrode 100, in particular relatively high rates of rotation, results in electrode 100 bowing and/or flexing radially outward in the region proximate of bushing 104. The greater the rate of rotation of electrode 100 the greater the degree of deflection of distal portion 110b and the greater the deviation from linearity of rotating electrode 100 from an idle electrode 100. Since contact length “L1” acts like a pivot point about which distal portion 110b deflects the greater the degree of deflection of electrode 100 in the region proximal of bushing 104 the larger the deviation “Y1” which exists at the distal tip of electrode 100. Deviation “Y1” directly affects the geometry, accuracy and surface quality of the machined work piece.
Accordingly, the need exists for an improved out flushing guide bush capable of reducing the eccentricity of the electrode and which exhibits improved flushing performance.